One way to compensate for non-linear distortion components (hereinafter referred to as distortion components) produced in a power amplifier is predistortion, in which a predistorter generates distortion compensation components that cancel distortion components produced in the power amplifier. The distortion compensation components are added to a signal input into the power amplifier in advance to compensate for the distortion components.
It is known that power amplifiers in general produce frequency-dependent, complex distortion components when they are operating near their saturation regions where they provide high efficiency. Some power series digital predistorters that compensate for frequency-dependent distortion components include a frequency characteristic compensator (For example, S. Mizuta, Y. Suzuki, S. Narahashi, and Y. Yamao, “A New Adjustment Method for the Frequency-Dependent IMD Compensator of the Digital Predistortion Linearizer,” IEEE Radio and Wireless Symposium 2006, January 2006, pp. 255-258; and J. Ohkawara, Y. Suzuki, S. Narahashi, “Fast Calculation Scheme for Frequency Characteristic Compensator of Digital Predistortion Linearizer,” VTC-2009 Spring, April 2009).
FIG. 1 illustrates an example of a conventional-art power series digital predistorter 500 combined with an amplifier 10 and a feedback signal generator 20, which are peripheral devices. I-phase and Q-phase digital signals are input into the power series digital predistorter 500.
The power series digital predistorter 500 includes a linear transmission path 110, a third-order distortion generating path 520, a divider 130, a combiner 140, a digital-to-analog (DA) converter 150, an analog-to-digital (AD) converter 160, a monitor 170, and a controller 580. The linear transmission path 110 includes a delay circuit. The third-order distortion generating path 520 includes a third-order distortion generator 521, a third-order vector adjuster 522, and a third-order frequency characteristic compensator 523. The divider 130 distributes I-phase and Q-phase digital signals input into the power series digital predistorter 500 to the linear transmission path 110 and the third-order distortion generating path 520. The combiner 140 combines a signal output from the linear transmission path 110 with a signal output from the third-order distortion generating path 520. The DA converter 150 converts a digital signal output from the combiner 140 to an analog signal. The AD converter 160 converts I- and Q-phase analog signals output from the feedback signal generator 20, which extracts a part of a signal output from the amplifier 10 as a feedback signal, to I- and Q-phase digital signals, respectively. The monitor 170 measures the power of signals input into the power series digital predistorter 500 and amplified by a power amplifier 13 from signals output from the AD converter 160 and also measures the power of distortion components in each of any predetermined frequency bands that were produced in the power amplifier 13 from signals output from the AD converter 160. Based on the results of the measurement by the monitor 170, the controller 580 adjusts a third-order vector adjuster coefficient including a phase value and an amplitude value to be provided to the third-order vector adjuster 522 and adjusts a plurality of third-order frequency characteristic compensator coefficients each including a plurality of phase values and a plurality of amplitude values to be provided to the third-order frequency characteristic compensator 523.
The amplifier 10 includes a quadrature modulator 11, an up-converter 12 and the power amplifier 13. The quadrature modulator 11 quadrature-modulates I- and Q-phase analog signals. The up-converter 12 converts the frequency of a signal output from the quadrature modulator 11 to a predetermined frequency. The power amplifier 13 amplifies the power of a signal output from the up-converter 12. The amplified signal is provided from the output end of the power amplifier 13 to an antenna through a duplexer, not shown, for example. The feedback signal generator 20 includes a directional coupler 21, a down-converter 22, and a quadrature demodulator 23. The directional coupler 21 extracts a part of a signal output from the amplifier 10. The down-converter 22 converts the frequency of the signal extracted by the directional coupler 21 to a predetermined frequency. The quadrature demodulator 23 demodulates a signal output from the down-converter 22 to I- and Q-phase analog signals.
The third-order distortion generator 521 cubes signals output from the divider 130 to generate third-order distortion components. The third-order vector adjuster 522 multiplies the third-order distortion components generated by the third-order distortion generator 521 by third-order vector adjuster coefficients provided from the controller 580 to adjust the phase and amplitude of the third-order distortion components. The third-order frequency characteristic compensator 523 multiplies third-order distortion component upper sub-band and third-order distortion component lower sub-bands, in total M sub-bands, as illustrated in FIG. 2, by different third-order frequency characteristic compensator coefficients, respectively. The transmission signal band in FIG. 2 includes input signals of the power series digital predistorter 500 that passed through the amplifier 10. FIG. 3 illustrates an exemplary configuration of the third-order frequency characteristic compensator 523. The third-order frequency characteristic compensator 523 includes a serial-to-parallel conversion part 523a, a J-point Fast Fourier Transformation (FFT) part 523b, J (J≧M) complex multiplication parts 523cj (j is an integer from 1 to J), a J-point Inverse Fast Fourier Transformation (IFFT) part 523d, and a parallel-to-serial conversion part 523e. The serial-to-parallel conversion part 523a converts a serial signal output from the third-order vector adjuster 522 to parallel signals. The J-point FFT part 523b transforms each of the signals output from the serial-to-parallel conversion part 523a from time domain to frequency domain. A signal component outputted from the J-point FFT part 523b that is in sub-band 1 out of the M sub-bands is input into the complex multiplication part 523cj in which the third-order frequency characteristic compensator coefficient for sub-band 1 provided from the controller 580 is set. The complex multiplication part 523cj multiplies the input signal by the third-order frequency characteristic compensator coefficient to adjust the phase and amplitude of the signal and outputs the adjusted signal. The same processing is applied to sub-bands 2 to M. Of the signals output from the J-point FFT part 523b, signal components that fall in none of the M sub-bands are input into the J-point IFFT part 523d without being multiplied by a third-order frequency characteristic compensator coefficient in the complex multiplication part 523cj. The J-point IFFT part 523d transforms each of the signals output from the complex multiplication parts 523cj from frequency domain to time domain. The parallel-to-serial conversion part 523e converts parallel signals output from the J-point IFFT part 523d to a serial signal.
The controller 580 adjusts the third-order vector adjuster coefficients to be provided to the third-order vector adjuster 522 and the third-order frequency characteristic compensator coefficients to be provided to the third-order frequency characteristic compensator 523 so as to minimize distortion components produced in the power amplifier 13. The third-order frequency characteristic compensator coefficients are calculated as follows. FIG. 4 illustrates an exemplary process flow for calculating a third-order frequency characteristic compensator coefficient (phase value) that minimizes a distortion component in a sub-band m. While FIG. 4 illustrates a process flow for calculating a phase value, an amplitude value can be calculated by a process similar to the process in FIG. 4. Here, the phase value to be provided to the complex multiplication part 523cj for the sub-band m is a variable Xm, a distortion component is measured T1 times with different phase values, and a phase value Xm.t1 is provided to the complex multiplication part 523cj for the sub-band m for measuring the distortion component (where t1=0, 1, . . . , T1−1). T1 different values of Xm.t1 are determined beforehand, where T1 is greater than or equal to 3. The controller 580 sets the phase value Xm.t1 of the sub-band m in the complex multiplication part 523c, for the sub-band m (“SET PHASE VALUE” in FIG. 4). The monitor 170 measures distortion component power Dm.t1 of the sub-band m at the phase value Xm.t1 (“MEASURE DISTORTION COMPONENT” in FIG. 4). The phase value Xm.t1 and the distortion component power Dm.t1 are recorded (“RECORD MEASURED VALUE” in FIG. 4). The sequence from SET PHASE VALUE to RECORD MEASURED VALUES is repeated T1 times. T1 sets of Xm.t1 and Dm.t1 obtained through the repetitive process are used to find coefficients (a2.m, a1.m, a0.m) of a quadratic function (Dm=a2.mXm2+a1.mXm+a0.m) representing dependence of Dm on the phase values of frequency characteristic compensator coefficients by the method of least squares, where Dm is a variable of the distortion component in the sub-band m (“FIND COEFFICIENTS (a2.m, a1.m, a0.m)” in FIG. 4). Then, a phase value Xm.cal (=−a1.m/2a2.m) that minimizes the found quadratic function is calculated (“CALCULATE MINIMUM VALUE (Xm.cal=−a1.m/2a2.m)” in FIG. 4) and the calculated phase value is set in the complex multiplication part 523c for the sub-band m as the phase value of the third-order frequency characteristic compensator coefficient for the sub-band m (“SET CALCULATED VALUE” in FIG. 4). Then the process proceeds to amplitude value calculation. The amplitude value is calculated in a way similar to the way that the phase value was calculated and the calculated amplitude value is set in the complex multiplication part 523c. Then the process proceeds to calculation of the phase and amplitude values in the next sub-band m+1.
The method for calculating third-order frequency characteristic compensator coefficients that minimize distortion components described above can also be used to calculate third-order vector adjuster coefficients that minimize distortion components.